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Abstract:

The invention relates to a process for recovering valuable metals from a
superalloy which has the steps of digesting the superalloy in a salt
melt. The salt melt contains 60-95% by weight of NaOH and 5-40% by weight
of Na2SO4.

Claims:

1-21. (canceled)

22. A process for recovering valuable metals from a superalloy which
comprises digesting the superalloy in a salt melt containing 60-95% by
weight of NaOH and 5-40% by weight of Na2SO.sub.4.

23. The process according to claim 22, which further comprises adding
sodium carbonate in an amount not to exceed 10% by weight of the salt
melt.

24. The process according to claim 23, wherein the salt melt contains
75-90% by weight of NaOH, 5-20% by weight of Na2SO4 and 5-10%
by weight of sodium carbonate.

25. The process according to claim 22, wherein the superalloy contains one
or more of the metals from the group consisting of Ni, Co, Cr or Al as a
main component and one or more of the elements from the group consisting
of Re, Mo, Ta, Nb, W, Hf or Pt as secondary component.

26. The process according to claim 25, wherein the superalloy contains 0.5
to 12% by weight of rhenium.

27. The process according to claim 22, wherein at least 1 kg of the salt
melt is used per 1 kg of superalloy.

28. The process according to claim 22, wherein the digesting is carried
out in a moving melt.

29. The process according to claim 23, wherein the digesting is carried
out in a rotary tubular kiln operated batchwise or continuously.

30. The process according to claim 22, which further comprises passing air
and/or oxygen or a mixture thereof into the melt.

31. The process according to claim 22, which further comprises adding
oxidizing component to the melt, wherein the oxidizing component is a
nitrate, peroxodisulphate, peroxide of the alkali metal and/or mixtures
thereof.

32. The process according to claim 31, wherein 5 to 25% by weight of the
oxidizing component, based on the salt melt, are added to the melt.

33. The process according to claim 30, wherein the mixture of air and
oxygen consisting of 25 to 95% by volume of air and 5 to 75% by volume of
oxygen is passed into the melt.

34. The process according to claim 32, wherein the digesting is carried
out at temperatures of 800 to 1200.degree. C.

35. The process according to claim 32, wherein the superalloy is partly
oxidized.

36. The process according to claim 22, wherein three fractions consisting
of:water-soluble alkali metal oxometallate of the metals of the 6th
and/or 7th subgroup and/or of the 3rd main group of the Periodic Table of
the Elements and/or mixtures thereof;water-insolble components from the
group consisting of the metals Co, Ni, Fe, Mn or Cr and/or mixtures
thereof,oxide and/or water-insoluble alkali metal oxometallate of the
metals of the 4th or 5th subgroup of the Periodic Table of the Elements
and/or mixtures thereof are pre-formed in the melt.

37. A process for recovering valuable metals from a superalloy comprising
the following steps:a) converting of the melt digestion product according
to claim 36 into the solid phase by cooling to room temperature,b)
commination of the solidified melt digestion product,c) reacting of the
comminuted melt digestion product in water at temperatures of less than
80.degree. C. and production of an aqueous suspension containinga
solution consisting of a mixture of sodium compounds from the group
consisting of NaOH, Na2SO4, NaAl(OH)4 and/or
Na2CO3 and alkali metallates of the elements of the 6th and/or
7th subgroups of the Periodic Table of the Elements;a solid metallic
phase consisting of the group of metals Co, Ni, Fe, Mn and Cr;a solid
phase consisting of hydroxides and/or hydrated oxides of the metals of
the 3rd main group and of metals of the 4th and/or 5th subgroup of the
Periodic Table of the Elements,d) removing of the aqueous fraction by
filtration,e) separating of the water-insoluble fraction by magnetic
deposition of metallic components, andf) removing the oxidic fraction.

38. The process according to claim 37, wherein the reaction of the melt
digestion product in water is carried out at temperatures of less than
60.degree. C.

39. The process according to claim 37, wherein the reaction of the melt
digestion product in water is carried out at temperatures of less than
40.degree. C.

40. A process for obtaining rhenium from a superalloy consisting of the
following steps:a) digesting the superalloy in a salt melt consisting
essentially of 60-95% by weight of NaOH and 5-40% by weight of
Na2SO4,b) cooling of the melt to room temperature,c)
commination of the melt digestion product,d) reacting of the comminuted
melt digestion product in water at temperatures of less than 80.degree.
C. and production of an aqueous suspension containinga solution
consisting of a mixture of sodium compounds from the group consisting of
NaOH, Na2SO4, NaAl(OH)4 and/or Na2CO3 and alkali
metallates of the elements of the 6th and/or 7th subgroup of the Periodic
Table of the Elements;a solid metallic phase consisting of the group of
metals Co, Ni, Fe, Mn and Cr;a solid phase consisting of hydroxides
and/or hydrated oxides of the metals of the 3rd main group and of metals
of the 4th and/or 5th subgroup of the Periodic Table of the Elements,e)
removing of the aqueous fraction by filtration, andf) removing of the
rhenium from the aqueous fraction.

41. The process according to claim 40, which further comprises adding
sodium carbonate in an amount not to exceed 10% by weight of the salt
melt.

42. The process according to claim 22, wherein the superalloy is a
superalloy scrap.

Description:

[0001]The present invention relates to a process for the digestion of
superalloys, in particular superalloy scrap, in a salt melt and
subsequent recovery of the valuable metals.

[0002]Superalloys are alloys which have a complex composition, are stable
at high temperatures and are based on nickel and cobalt, with additions
of other metals, such as, for example, aluminium, chromium, molybdenum,
tungsten, tantalum, niobium, manganese, rhenium, platinum, titanium,
zirconium and hafnium, and nonmetals, such as boron and/or carbon. The
superalloys are high-strength and particularly hard-wearing alloys which
are used in motor and engine construction, in energy technology and in
aviation and space flight. The particular properties of these alloys are
achieved in particular by the addition of rare and noble metals, such as
rhenium, tantalum, niobium or even platinum. A good overview of the
composition, properties and fields of use of the superalloys is to be
found in Ullmann's Encyclopedia of Industrial Chemistry, Volume A13,
Fifth Edition, 1989, pages 55-65, and in Kirk-Othmer Encyclopedia of
Technology, Volume 12, Forth Edition, pages 417-458.

[0003]The superalloys differ from the customary high-melting alloys, e.g.
W--Re alloys or Mo--Re alloys, in their particular resistance to
oxidation or corrosion. Thus, owing to their excellent oxidation
stability, components comprising superalloys are used in the production
of blades in aircraft turbines. After elapse of the duration of use, such
parts are an important raw material source for recovering rare metals, in
particular rhenium, tantalum, niobium, tungsten, molybdenum and platinum.

[0004]The recovery of the alloy metals of the superalloys is commercially
very interesting owing to the high proportion of expensive metals. Thus,
special superalloys contain the metals rhenium in up to 12% by weight,
tantalum in up to 12% by weight, niobium in up to 5% by weight and
tungsten and molybdenum in up to 12% by weight. Further metals which
serve as base metals in the superalloys are nickel and cobalt. For the
last-mentioned metals, too, the superalloys are a raw material source
from which the recovery of these metals is commercially expedient.

[0005]For the recovery of the metallic components from superalloys, a
large number of hydrometallurgical or pyrometallurgical and
electrochemical processes are known which, owing to their complex
embodiments and high energy demand, are not processes which are not
carried out on a large scale from commercial points of view, especially
owing to the constantly increasing energy prices.

[0006]According to the prior art, for the recovery of the metallic
components from the superalloys, the latter are melted kept under an
inert gas atmosphere and then atomized to give a finely divided powder.
In this procedure, a disadvantage is that the superalloys melt only at
high temperatures between 1200 and 1500° C. The actual digestion
of the superalloy takes place only in a second step by treatment of the
powder obtained with acids. Experience has shown that several days are
required for this purpose. According to another process, clump-like
superalloy scrap is first comminuted by energy-intensive milling
processes after prior embrittlement, for example at low temperatures, and
then digested by a wet-chemical method at elevated temperatures in
mineral acids of a certain concentration and composition, Potter et al.,
Eff. Technol. Recycling Metal 1971, page 35 et seq.

[0007]Furthermore, some processes which envisage the digestion of the
superalloy scrap via electrochemical processes are known.

[0008]According to U.S. Pat. No. 3,649,487, the high-melting metals
present in scraps of an Fe/Ni/Co/Cu base alloy, e.g. tungsten, molybdenum
and chromium, are first converted into borides, carbides, nitrides,
silicides or phosphides via a melting process by addition of non-metallic
compounds of group III, IV or V, melted to give anodes and then subjected
to an anodic oxidation. Those metals such as Co, Ni and Cu initially go
into solution and are deposited from this at the cathode, while the
high-melting metals, remain behind in the anode sludge, for example as
borides, carbides, etc. It is disclosed here that the metals Ni, Co, Cu
are separated from the high-melting metals, such as W, Mo or chromium,
but there is no information at all about whether complete separation of
these metals takes place. The document furthermore provides no
information about the cost-efficiency of the process.

[0009]WO 96/14440 describes a process for the electrochemical digestion of
superalloys by anodic oxidation of the alloy in an electrolysis bath with
an organic solvent component. The document discloses that up to 10% of
water can be added to the electrolyte solution so that the process can
still be carried out according to the invention. Otherwise, passivation
of the anode occurs through formation of a gel or a firmly adhering oxide
layer, which can lead to termination of the electrolysis. The working-up
and separation of the valuable substances from the suspension forming as
a result of the electrolysis are initially effected by filtration. The
filtration residue separated off and containing a part of the alloy
metals is then worked up thermally by calcination and subsequently by the
customary hydrometallurgical processes.

[0010]DE 10155791C1 likewise discloses an electrochemical digestion
process for superalloys. In this process, the superalloys are first cast
into sheets and then electrolytically digested in an oxygen-free
inorganic acid. Here, the problem of anodic passivation is counteracted
by reversal of the polarity of the electrodes. The two last-mentioned
processes can be implemented economically only under certain general
conditions, in particular very high rhenium contents in superalloys.

[0011]DE 19521333 C1 discloses a pyrometallurgical digestion of
tungsten-containing hard metal and heavy metal scraps. The digestion
takes place at temperatures between 800 and 1000° C. in a salt
melt which consists of NaOH and Na2SO4. In these processes, a
sodium tungstate melt is produced, which is dissolved in water after
subsequent cooling.

[0012]As in the present invention, tungsten hard metal scrap is virtually
completely digested there in alkaline, sulphate-containing melt under
oxidizing conditions by formation of sodium tungstate. This is not
surprising since the metallate is distinguished by high stability and
dissolves in the NaOH melt under the reaction conditions. Thus, a
complete dissolution process of the hard metal scrap is ensured.

[0013]It was an object of this invention to provide a process for the
digestion and recycling of superalloys, in particular rhenium-containing
superalloy scraps, and working-up for recovery of the valuable materials
present therein as a more economical alternative to recycling by anodic
oxidation or acid digestion.

[0014]The object was achieved by a process for the recovery of valuable
metals from superalloys, the superalloys being digested in a salt melt
consisting of 60-95% by weight of NaOH and 5-40% by weight of
Na2SO4 and the melt digestion product formed thereby then being
worked up hydrometallurgically with the aim of simple separation of the
individual valuable metals.

[0015]The digestion is preferably carried out in a salt melt consisting of
65-85% by weight of NaOH and 15-35% by weight of Na2SO4,
particularly preferably of 70-80% by weight of NaOH and 20-30% by weight
of Na2SO4.

[0016]In the case of superalloys with the digestion of which the present
invention is concerned, more than over 50% of the metallic constituents,
e.g. nickel or cobalt, do not form metallates under the reaction
conditions of DE 19521333 C1, and it was surprising that a corresponding
digestion could take place at all. Furthermore, it was surprising that
virtually all the nickel and cobalt was present in metallic form after
digestion and hence particularly advantageous working-up of the melt
digestion product where the use of magnetic separation was possible. At
least, this results in a substantial economic advantage over the
electrochemical digestion processes cited for superalloys. Superalloys
according to the present invention are alloys which contain, as main
components, 50 to 80% of nickel, 3 to 15% by weight of at least one or
more of the elements cobalt, chromium and optionally aluminium, and 1 to
12% by weight of one or more of the elements rhenium, tantalum, niobium,
tungsten, molybdenum, hafnium and platinum.

[0017]The process according to the invention is suitable in particular for
rhenium-containing superalloys which contain up to 12% by weight of
rhenium. The digestion according to the invention of superalloys is
advantageously carried out in such a way that up to 10% by weight,
preferably up to 8% by weight and particularly preferably up to 5% by
weight of sodium carbonate (Na2CO3), based on the weight of the
salt melt, are added to the salt melt.

[0018]Advantageous compositions of the salt melt are listed in Table 1.

[0019]The superalloys may be present both in lump form and in pulverulent
form (grindings or grinding dusts).

[0020]The superalloy digestion can be carried out both in directly heated
furnaces, e.g. in furnaces with gas or oil firing, and in indirectly
heated furnaces, continuously or batchwise. The furnaces suitable for
this purpose are, for example, rotary furnaces and rotary tubular kilns.

[0021]The digestion of superalloys is preferably carried out in a moving
alkaline melt in a directly fired rotary tubular kiln operated batchwise.

[0022]The digestion according to the invention is carried out in such a
way that at least 1 kg of salt melt, preferably at least 1.5 kg and
particularly preferably at least 2 kg are used per 1 kg of superalloy. In
the case of certain superalloys which have rhenium contents greater than
8%, up to 5 kg of salt melt are used per kilogram of superalloy.

[0023]The digestion according to the invention of superalloys takes place
particularly advantageously with regard to the space-time yield if air
and/or oxygen, or a mixture thereof, is passed into the salt melt. A
mixture of air and oxygen consisting of 25 to 95% by volume of air and 5
to 75% by volume of oxygen, preferably of 35 to 80% by volume of air and
20 to 65% by volume of oxygen, is preferably passed into the salt melt.

[0024]The digestion according to the invention of superalloys is carried
out at temperatures of 800 to 1200° C.

[0025]Preferably, the digestion is carried out in the temperature range of
850 to 1100° C., particularly preferably at 900 to 1050° C.
Good digestion conditions are present if oxidizing agents are
additionally introduced into the melt. For example, nitrates,
peroxodisulphates, peroxides of the alkali metals and/or mixtures thereof
can serve as such. Potassium nitrate, sodium nitrate, sodium peroxide,
potassium peroxide, sodium peroxodisulphate, potassium peroxodisulphate
and/or mixtures thereof are advantageously used as oxidizing agents.
Particularly good digestion rates are achieved if 5 to 25% by weight of
the oxidizing component, based on the weight of the melt, are added to
the melt.

[0026]Advantageous compositions of the salt melt are shown in Table 2.

[0027]The melt digestion is particularly advantageously carried out in
such a way that a partial oxidation of the superalloy takes place or,
after virtually complete oxidation, reducing conditions are established
for a certain time. In the digestion process according to the invention,
three fractions are pre-formed in the melt itself, consisting of:
[0028]water-soluble alkali metal oxometallates of the metals of the 6th
and/or 7th subgroup and/or of the 3rd main group of the Periodic Table of
the Elements and/or mixtures thereof; [0029]water-insoluble components
from the group consisting of the metals Co, Ni, Fe, Mn or Cr and/or
mixtures thereof, [0030]oxides and/or water-insoluble alkali metal
oxometallates of the metals of the 4th or 5th subgroup of the Periodic
Table of the Elements and/or mixtures thereof.

[0031]These three fractions are then worked up hydrometallurgically. The
present invention therefore relates to a process for working up the
superalloy melt digestion product, comprising the following steps:

a) conversion of the melt digestion product into the solid phase by
cooling to room temperature,b) commination of the solidified melt
digestion product,c) reaction of the comminuted melt digestion product in
water at temperatures of less than 80° C. and production of an
aqueous suspension containing [0032]a solution consisting of a mixture
of sodium compounds from the group consisting of NaOH, Na2SO4,
NaAl(OH)4 and/or Na2CO3 and alkali metallates of the
elements of the 6th and/or 7th subgroups of the Periodic Table of the
Elements; [0033]a solid metallic phase consisting of the group of metals
Co, Ni, Fe, Mn and Cr; [0034]a solid phase consisting of hydroxides
and/or hydrated oxides of the metals of the 3rd main group and of metals
of the 4th and/or 5th subgroup of the Periodic Table of the Elements,d)
removal of the aqueous fraction by filtration,e) separation of the
water-insoluble fraction by magnetic deposition of metallic components,f)
removal of the oxidic fraction.

[0035]The process according to the invention is shown schematically in the
attached FIG. 1. According to FIG. 1, the superalloy melt digestion
product (2) is crushed after cooling to room temperature, then comminuted
in a mill and then leached in water. Preferably, the leaching is carried
out at temperatures of less than 60° C. and particularly
preferably at less than 40° C. The particular feature of the melt
digestion comprises the three fractions which are formed therein
beforehand and are present during the water leaching as fractions which
can be easily separated: [0036]the filtrate (4) which substantially
contains the elements molybdenum, tungsten and rhenium in the form of
their alkali metallates, [0037]the water-insoluble residue (3) which
consists of a magnetic fraction which contains practically the total
nickel and cobalt fractions of the alloy and about 1/3 of the chromium
used, in metallic form, while all other elements are present only as
secondary constituents or in the trace range, and [0038]a nonmagnetic
fraction (5) which contains the elements aluminium, chromium, titanium,
zirconium, hafnium, niobium and tantalum in the form of their oxides
(e.g. Al2O3, Cr2O3, TiO2, ZrO2, HfO2, Ta2O5, Nb2O5), or hydroxides
(e.g. Al(OH)3, Cr(OH)3, Ti(OH)4, Zr(OH)4, Hf(OH)4, Ta(OH)5, Nb(OH)5
or nitrides (e.g. AlN, CrN, TiN, HfN, NbN and TaN) or carbides (e.g. AlC,
Cr2C3, TiC, ZrC, HfC, NbC and TaC).

[0039]The further working-up of these fractions can be effected by the
known methods. Thus, the rhenium can be separated off after the
filtration from the filtrate (4) over strongly basic ion exchangers, as
described in DE 10155791. The rhenium-free solution containing
substantially sodium molybdate and sodium tungstate can be added to the
process for obtaining molybdenum and tungsten.

[0040]The nonmagnetic residue, which contains up to 15% of tantalum, can
be used as raw material in tantalum-metallurgy.

[0041]The magnetic residue is advantageously used for the production of
cobalt and nickel.

[0042]The process according to the invention is suitable in particular for
recovering rhenium from superalloys. The present invention furthermore
relates to a process for obtaining rhenium from superalloys, comprising
the following steps:

a) digestion of superalloys in a salt melt consisting of 60-95% by weight
of NaOH and 5-40% by weight of Na2SO4,b) cooling of the melt to
room temperature,c) commination of the melt digestion product,d) reaction
of the comminuted melt digestion product in water at temperatures of less
than 80° C. and production of an aqueous suspension containing
[0043]a solution consisting of a mixture of sodium compounds from the
group consisting of NaOH, Na2SO4, NaAl(OH)4 and/or
Na2CO3 and alkali metallates of the elements of the 6th and/or
7th subgroup of the Periodic Table of the Elements; [0044]a solid
metallic phase consisting of the group of metals Co, Ni, Fe, Mn and Cr;
[0045]a solid phase consisting of hydroxides and/or hydrated oxides of
the metals of the 3rd main group and of metals of the 4th and/or 5th
subgroup of the Periodic Table of the Elements,e) removal of the aqueous
fraction by filtration,f) removal of the rhenium from the aqueous
fraction according to DE 10155791.

[0046]The process according to the invention for obtaining rhenium from
superalloys is advantageously carried out in a manner such that up to 10%
by weight, preferably up to 8% by weight and particularly preferably up
to 5% by weight of sodium carbonate (Na2CO3), based on the
weight of the salt melt, are added to the salt melt. The removal of the
rhenium from the aqueous suspension by means of strongly basic ion
exchange resins is preferred.

[0047]An advantage of the process according to the invention is that the
superalloy digestion in an NaOH--Na2SO4 melt is exothermic. By
passing in air or an air/oxygen mixture, the process is readily
controllable. A further advantage is that the valuable substances can be
virtually completely recovered.

[0048]The invention is explained in more detail with reference to the
following example.

EXAMPLE

[0049]1.97 t of superalloy grinding dust (1) were heated together with
2.50 t of NaOH and 0.45 t of Na2SO4 to 1110° C. in the
course of 4 hours in a rotary furnace directly fired with natural gas and
left at this temperature for a further hour. The composition of the
superalloy grinding dust is shown in Table 1.

[0050]Thereafter, the resulting viscous superalloy melt digestion product
was completely poured out of the furnace. The cooled melt was first
coarsely crushed and then melted to <2 mm. 5.26 t of pulverulent melt
material (2) were obtained, which material was stirred into 7.5 m3
of water for leaching. After the end of the addition, stirring was
continued for a further 2 hours, followed by filtration over a filter
press and rinsing with 0.5 m3 of water. 2.10 t of filter residue (3)
and 9.3 m3 of filtrate (4) were obtained. The filter cake was
suspended again in water, and the metallic, magnetic fractions were
separated from the oxidic and hydroxidic fractions by circulating the
suspension through a magnetic separator by means of a pump. The
substantially metal-free suspension was then separated again by means of
a filter press, and the filtrates were initially introduced for the next
leaching run. 1.46 t of metal sludge (5) and 0.56 t of hydroxide sludge
(6) were obtained. The hydroxide sludge (6) was sent to a tantalum
facility for recovering the tantalum, and the metal sludge (5) was sent
to a nickel facility for further working-up. The rhenium-containing
filtrate (3) was passed over ion exchange columns with strongly basic ion
exchangers for recovering the rhenium. The further enrichment and
purification of the rhenium were effected by standard methods according
to the prior art. The rhenium-free outflow of the ion exchange columns
was used in a tungsten facility as an initially taken material for the
leaching of WO3. The rhenium yield was 94%.

[0051]The composition of the superalloy grinding dust and of the most
important intermediates is shown in Table 3.